Photovoltaic devices, also referred to as solar cells, convert light directly into electricity. The majority of photovoltaic devices use a semiconductor as an absorber layer with a well-defined bandgap, such as crystalline silicon with an energy bandgap Eg of 1.1 eV. Photovoltaic devices include layers of semiconductor materials with different electronic properties. One of the layers of silicon can be “doped” with a small quantity of boron to give it a positive (or p-type) character. Another layer can be doped with phosphorus to give it a negative (or n-type) character. The p and n regions can be adjacent to each other or separated by an intermediate layer. The interface, or junction, between these two layers contains an electric field.
When light (i.e., photons) hits the device, some of the photons are absorbed in the region of the junction, freeing electrons and holes (i.e., carriers) in the absorber. If the photons have enough energy, the carriers will be driven out by the electric field and move through the silicon and into an external circuit.
Light with energy lower than the bandgap is not absorbed and is thus lost for photoelectric conversion. Light with energy E greater than the bandgap Eg is absorbed. However, the excess energy E−Eg is lost due to thermalization. It is well known that this results in an optimum choice for the bandgap of the absorber material. Invoking the principle of detailed balance, the optimum bandgap of a photovoltaic device has been found to be about 1.4 eV with a limiting conversion efficiency of 33%.
In single bandgap cells, only a fraction of the energy spectrum of the incident light is used for the energy conversion. For example, only a part of the energy of incident sunlight is available for photo conversion.
Amorphous materials have been proposed for use in photovoltaic devices. Known amorphous silicon photovoltaic designs are limited in conversion efficiency owing to high recombination rates. This problem is primarily due to the presence of high numbers of defect carrier traps situated deep within the bandgap. Such traps forbid the efficient transfer, and thus separation of electric charges resulting in low carrier mobility. Two main sources of the defects include hydrogen microstructure, and incomplete dopant activation. In the case of the latter, the effect is responsible for poor transport characteristics widely seen in p-doped microcrystalline silicon. In fact, because of the general inability of sputter processing to generate microcrystallinity in thin film silicon, the dopant activation is virtually nil.
Another issue related to amorphous silicon designs is the inability to control the bandgap via alloy addition. This limits the possible capture of photons to those with energies greater than 1.8 eV. Amorphous germanium is an ideal candidate to alloy with the silicon since the bandgap is about 1.0 eV. However, although silicon and germanium are miscible, a problem arises during fabrication associated with the preferred deposition technique, chemical vapor deposition (CVD). Competitive reaction rates lead to poor optoelectronic properties with increasing germanium concentration. The industry-wide solution thus far has been to control the bandgap with partial crystallinity since the presence of crystal silicon leads to a lower bandgap, since the bandgaps are approximately 1.8 eV for amorphous silicon, and 1.1 eV for crystalline silicon. While this approach is effective, it necessitates thicker absorber regions since the increased crystalline content drastically decreases the effective absorption coefficient of the material.